1. Field of the Invention
The present invention is directed to monitoring and controlling wellbore operations, and, in one particular aspect, to monitoring and controlling wellbore drilling operations in real time.
2. Description of Related Art
The prior art discloses a wide variety of systems and methods for monitoring wellbore operations and for sensing and measuring parameters related to such operations, both downhole and at the surface. The prior art also discloses a wide variety of sensors, measurement apparatuses, devices, and equipment for sensing, measuring, recording, displaying, calculating, processing, and transmitting measured values for operational parameters, including, but not limited to, weight on bit (WOB), rate of penetration (ROP), rotary speed, bit speed, top drive speed, downhole motor speed, and torque on a drillstring or on a bit.
Many systems and methods have been proposed and implemented for using such sensed and measured operational parameters to enhance, facilitate, and, in some cases, optimize operational performance and the performance of apparatuses, devices and equipment involved in such operations; including, but not limited to, drilling operations. In 1965 R. Teale proposed a model for analyzing and predicting drilling performance based on a calculation of “mechanical specific energy” in an article entitled “The Concept Of Specific Energy In Rock Drilling” [Int'l J. Rock Mech. Mining Sci. (1965) 2, 57-73]. Teale's mathematical definition (“Teale definition”) of mechanical specific energy, Es, is:
  Es  =            WOB      A        +                  120        ⁢                                  ⁢                  π          ⁡                      (            N            )                          ⁢                  (          T          )                            A        ⁡                  (          ROP          )                    In which WOB is weight on bit, N is rpm's of a rig's rotary, T is the torque at the bit, ROP is rate of penetration, and A is wellbore (or bit) cross-sectional area.
In a 1992 research study, (see paper entitled “Quantifying Common Drilling Problems With Mechanical Specific Energy And A Bit-Specific Coefficient of Sliding Friction”, SPE 24584, 373-388), R.C. Pessier et al developed an energy balance model for drilling under hydrostatic pressure using a comparison between full-scale simulator tests and field data. As key indices of drilling performance, they employed mechanical efficiency, Teale's mechanical specific energy parameter, and a bit-specific coefficient of sliding friction for bit selection and analysis. “Mechanical specific energy” was defined as work done per unit volume of rock drilled and it was assumed that the minimum specific energy required to drill is approximately equal to the compressive strength of the rock being drilled. The mechanical efficiency of drilling was then estimated by comparing actual specific energy required to drill an interval with the minimum expected specific energy needed to drill that interval. Pessier et al analyzed values of various parameters (actual specific energy, minimum specific energy, energy efficiency, and bit-specific coefficient of sliding friction) with respect to ROP under different situations (e.g., different bits, different WOB's, different RPM's, different hydraulics, and under atmospheric and hydrostatic pressure). It was concluded that mechanical specific energy, mechanical efficiency, and bit-specific coefficient of sliding friction provided good indicators of drilling performance and could enhance the interpretation of data for: the detection and correction of major drilling problems; analysis and optimization of drilling practices; bit selection; failure analysis; evaluation of new drilling technologies and tools; real-time monitoring and controlling of the drilling process; analysis of MWD (measurement while drilling) data; and further system developments.
In a 2002 paper, Waughman et al reported on a system and method for optimizing the bit replacement decision [“Real-Time Specific Energy Monitoring Reveals Drilling Inefficiency and Enhances the Understanding of When to Pull Worn PDC Bits,” IADC/SPE 74520, 2002, 1-14]. The system involved measuring the mechanical energy input at the drill rig floor, calculating the drilling specific energy, checking current formation type via real-time downhole gamma ray readings, comparing the specific energy with the benchmark new bit specific energy, and then using these values to assess the bit's “dull” state. Success of the system was reported for synthetic based mud systems Where bit balling does not mask bit dull condition. The process worked in water-based drilling fluids that had replaced earlier synthetic muds because both balled-new bits and dull bits exhibit similar levels of inefficiency.
In general, many prior art systems and methods use undifferentiated mechanical specific energy, i.e., calculations of mechanical specific energy based on sensed and measured values without taking into account the location of the sensors and measurement apparatuses that produce them. No discrimination is made for data obtained from downhole as opposed to surface locations. No differentiation is made between data obtained from locations at the bit as opposed to in the drillstring or at the surface. For example, torque and rotational speed (rpm's) can be measured at various locations—e.g. downhole or at the surface, and the measurement, from whichever location, is then used. The use of such undifferentiated measurements or parameters such as torque, rotational speed, etc. can lead to ambiguous and/or inconsistent determinations of mechanical specific energy.
There is a need, recognized by the present inventors, for efficient and effective systems and methods for monitoring and controlling wellbore operations, and, in one aspect, in which such operations are drilling operations.
There is a need, recognized by the present inventors, for such systems and methods which employ localized and accurate determined values for mechanical specific energy.